Manufacturers of titanium sponge or ingots have recently been inundated with requests for production increase to satisfy greater demand of titanium metal. Not only manufacturers of titanium sponge or ingots, but also manufacturers that process titanium ingots into forged plate material, are in a similar situation.
A conventional general process for the production of a strip coil, which is a kind of plate material processed from the titanium ingot mentioned above, involves first melting titanium raw material by a consumable electrode type arc melting method or an electron beam melting method, solidifying the melted metal as a large titanium ingot, and then breaking down the ingot into a slab for hot rolling.
This large ingot has a circular cross section having a diameter of about 1 m in the case of the consumable electrode arc melting method. In the case of the electron beam melting method, an ingot having a rectangular cross section can also be produced, and width of the rectangular cross section is about 0.5 to 1 m. Since the ingots have large cross section, these large ingots are broken down by hot processes such as milling, forging, and rolling, to have a slab shape so as to be enabled to be rolled by a hot rolling machine.
After the breaking down, a process for reforming the deformations for the thickness direction and for the width direction (camber) and a process for removing scale and damage on the surface are applied, thereafter a slab for hot rolling can be obtained. This slab for hot rolling is to be heated to a predetermined temperature and hot-rolled by a common hot rolling machine for steel or the like, into a strip coil (thin plate). After that, this hot rolled strip coil is to be annealed or descaled into a product, or is to be further cold-processed by such method as cold rolling and annealing into a product.
The cost for producing thin plate coil is accordingly increased with the number of production steps as mentioned above. Therefore, the manufacturer of the titanium ingot is required to provide a titanium slab which leads to shortening or improving the above steps.
On the other hand, recently, rectangular prism shape ingots have also been produced by making a mold having a rectangular cross section in an electron beam melting furnace. However, the thickness of the rectangular prism ingot is not small enough to be processed directly by the hot rolling machine without the breaking down process. Therefore, a process technology in which a thinner rectangular prism ingot can be produced is required; however, practical use in production has not yet been achieved.
That is, to produce a titanium slab having a thickness that can be directly fed into a hot rolling machine by using a conventional electron beam melting furnace, a specially designed mold to produce such a titanium slab is first required. However, in the case in which thickness of a conventional rectangular mold is simply reduced during the production of the titanium slab in the electron beam melting furnace, the titanium slab produced by the mold would have deformations for the thickness direction and for the width direction and would be wavy along the longitudinal direction. In such cases, the titanium slab cannot be directly used with conventional hot rolling machine used for rolling steel or the like.
When producing a strip coil by a conventional hot rolling machine for steel or the like, properties of the material going through the machine (linearity) would be impaired by the deformation of the slab, the material would be greatly deformed up and down or left and right, the material would not pass through straight, and continuous hot rolling could no longer be performed. Even if hot rolling was performed, since the rolled material would strike a guide or a feeding roll, the edge part would be cracked or the surface would be damaged. In a case in which the deformation of the produced titanium slab is significant, it would be necessary for the material to be processed and corrected by heating and or by grinding to remove a certain portions from the material in the thickness or width direction.
A process for production of rectangular prism ingot using an electron beam melting furnace having a rectangular mold is disclosed in Patent Document 1, for example. FIG. 1 of this publication discloses a situation in which molten metal is poured from a mold wall of the longer. The Patent Document 1 discloses an effect in which the rectangular prism ingot is produced to improve rolling processing of the ingot; however, there are no technical disclosures concerning linearity of the ingot in such terms as deformation of the titanium slab produced by the rectangular mold.
However, upon considering existing production processes, a process technology in which a titanium slab produced in an electron beam melting furnace under reduced pressure is drawn out at atmospheric pressure has not yet been in practical use. To draw out the slab, the electron beam irradiation should be stopped and the inside of the furnace should be held at atmospheric pressure, thus it is difficult to perform the electron beam melting process and the process of drawing out the slab continuously.
As mentioned above, to directly produce a titanium slab appropriate for hot rolling by an electron beam melting furnace, it is necessary to reasonably solve the above-mentioned matters.
The Patent Document 2 discloses a method in which a titanium slab is drawn out of a mold of an electron beam melting furnace, an electron beam is irradiated to heat and melt the surface thereof, and the slab is rolled by a surface shaping roll, so as to improve the surface of the casted slab.
According to Patent Document 2, since there is surface damage or large oscillation marks in the case in which the slab is merely drawn out of the mold, an electron beam, in the subsequent steps, is again irradiated to melt the surface, and the slab is rolled by the surface shaping roll to obtain a good casting surface. A sample of a rectangular prism titanium slab having a cross section of 180 mm×50 mm is exemplified.
However, Patent Document 2 does not disclose a technique concerning linearity of the produced material, such as deformations for the thickness direction and for the width direction of a titanium slab.
In addition, the cross section of 180 mm×50 mm as described is too small to be processed by an industrial scale hot rolling machine such as for steel to produce a strip coil.
Furthermore, in the Patent Document 2, it is necessary to further prepare the surface shaping roll and the electron gun for heating the titanium slab in addition to an electron beam melting furnace after the slab is drawn out of the mold, and thus, there are cost issues to be solved.
Furthermore, recently, a technique in which a rectangular mold is arranged in an electron beam melting furnace to produce a rectangular ingot has been developed. A rectangular prism ingot is easier to be hot forged compared to a round-shaped ingot, and thus efficiency of the forging process can be improved.
Furthermore, a process for production of a slab in which thickness of the ingot is further reduced has been researched; however, the slab produced have cracks or damages at corners thereof, and thus it is necessary to improve the situation.
In the case in which the slab is cracked or damaged, the damage may remain at the surface of a thin plate after subsequent forging or rolling processing, or the thin plate itself may be cracked. Furthermore, even if there are no cracks or damages at corners, edges may be cracked during hot rolling in the case in which the shape of the corner of the rectangular slab is not appropriate. In this case, yields of the thin plate product may be greatly reduced and improvement to overcome these problems has been required.
In this regard, a test in which cooling intensity at corners of the slab is lessened by subjecting inside parts of a mold to the outside observed in continuous casting technique, to produce an ingot having an improved surface, is disclosed in Patent Document 3.
Furthermore, a technique in which a cross section of the mold is formed so as to decrease along the pulling direction of the slab to improve fitting property of the mold and the slab, to improve the corner parts and surface of the slab, is disclosed in Patent Document 4.
However, these techniques concern improvement of the surface of the entire cast body, and problems about cracks generated at the corners of the rectangular ingot are neither disclosed nor suggested. As explained, a technique has been required in which a rectangular ingot having a good surface and not having cracks or damage at the corners produced by the electron beam melting furnace, can be reliably produced.
An object of the invention is to provide a titanium slab having properties suitable for hot rolling, which can be directly fed into a hot rolling machine without a breaking down process or subsequent correcting process after melting in an electron beam melting furnace, and to provide a process for production thereof.    Patent Document 1: Japanese Patent Application, Laid Open Publication No. Hei 04 (1992)-131330    Patent Document 2: Japanese Patent Application, Laid Open Publication No. Sho 62 (1987)-050047    Patent Document 3: Japanese Patent Application, Laid Open Publication No. Hei 11 (1999)-028550    Patent Document 4: Japanese Patent Application, Laid Open Publication No. Hei 04 (1992)-319044